1. Field of the Invention
The present invention relates to multiple frequency acoustic transducers used, especially, in medicine to form images of the human body by echography.
2. Description of the Prior Art
Prior art methods in medical echography include the use of probes. A cross-section of a probe is shown in FIG. 1. This probe is made up of aligned transducer elements 101, the thickness of which is adapted to the operating frequency. The two sides of these elements are lined with electrodes 102 used to apply the electrical voltages which make them vibrate. The vibration frequency chosen is most usually the resonance frequency F.sub.r corresponding to the fundamental vibration mode depending on the thickness of the transducer. For the piezoelectric materials generally used in these probes, the relationship between f.sub.r, expressed in kilohertz, and the thickness h, expressed in millimeters, is given by f.sub.r =2850/h. Usually, for medical probes, a thickness of 1 mm is used, and the frequency used is then most often 2.85 MHz.
The Q factor of the transducers is approximately equal to the ratio between the impedance of the piezoelectric material forming this transducer and the impedance of the external medium in which the vibration will be propagated. If .rho. and .rho..sub.o are the relative densities of the piezoelectric material and the external environment respectively, and if c and c.sub.o are the speeds of sound in this material and in this medium respectively, then Q is equal to .rho..sub.c /(.rho..sub.o c.sub.o). In the case of a piezoelectric ceramic, such as the PZT, this ratio is close to 17.
The vibrations are emitted in the form of brief pulses in order to obtain adequate definition in distance. This widens the frequency band of the signal emitted and therefore makes it necessary to have a relatively large band width for the probe. To obtain this, a strip 103 is placed in front of the transducers, the thickness of this strip being a quarter of the wavelength at the fundamental frequency. The impedance of this quarter wave-strip is chosen to be in the range of .sqroot..rho.c.rho..sub.o c.sub.o.
The transducers are fixed to the frame of the probe by means of a backing 104 which is advantageously of the soft type, i.e. with an acoustical impedance in the region of 0.
Two types of operation are habitually used in medical imaging:
standard imaging, called mode B imaging, where the echos are represented sectorially according to the aiming angle and distance, the amplitude of these echos modulating the brilliance of the image:
color-encoded imaging also called "Doppler flow mapping" or DFM where the Doppler shift due to blood circulation is represented by variations in color, in addition to variations in brilliance due to the amplitude of the echos.
For imaging in mode B, a high degree of lateral and distance definition is needed. This calls for a relatively high center frequency, for example, in the range of 5 MHz.
For DFM imaging, there is no need for definition as high as for mode B imaging, but the highest possible signal-to-noise ratio is needed to make it possible to measure small Doppler shifts themselves corresponding to low blood flow speeds. The signal-to-noise ratio is all the greater as the operating frequency is low. A typical value of the frequency used will be, for example, 2.5 MHz.
In the prior art, two probes connected to one and the same instrument are used, but this obviously increases the cost of the equipment and complicates its use. Another far less satisfactory method lies in the use of a single probe working at an intermediate frequency of 3.5 MHz for example.